The invention relates to quadrature modulators, and more particularly to the compensation of quadrature modulator amplitude imbalance.
Quadrature carrier transmitters use both inphase (I) and quadrature-phase (Q) carriers to transmit information. Quadrature modulation is required to upconvert the information bearing I- and Q-signals to an intermediate frequency (IF) or directly to the carrier frequency (direct conversion), depending on the structure of the transmitter radio frequency (RF) chain. A quadrature modulator operates on a quadrature modulation basis. It enables two independent signals to be combined in the transmitter and be transmitted on the same transmission band and the signals to be separated again at the receiver. The principle of quadrature modulation is that two separate signals, I and Q (Inphase and Quadrature phase), are modulated by using the same carrier wave frequency, but the phases of the carrier waves differ from each other in that the carrier wave of signal Q lags 90xc2x0 behind the carrier wave of signal 1. After modulation, the signals are summed. Thanks to the phase difference, the I- and Q-signals can be separated from each other when the sum signal is demodulated. When the quadrature modulation is implemented with analog components several distortions can affect the quality of the transmitted signal. The most common distortions are generally described as local oscillator or carrier leakage, quadrature phase error and amplitude imbalance. The distortions cause e.g. errors in the mutual phase and amplitude balance of the I- and Q-signals.
Semiconductor manufacturing processes do not produce perfect analog components. When a large amount of products are produced, there are always some variations in component values. If these variations are left uncorrected the quality of the transmitted signal will degrade and possibly not reach a level required by standards. This degradation can be mitigated in two ways, either by improving the manufacturing process, or by compensating the degradation in each individual product by using signal processing methods. Improving manufacturing process is expensive for large product quantities, generally more lax manufacturing specifications result in cost reductions, and hence more competitive product. Therefore signal processing methods are preferable as a method to compensate component value variations. Additionally quadrature modulator errors depend on temperature, aging, and carrier frequency. These effects are very difficult to compensate with the analog design of the quadrature modulator.
Correction circuits with fixed correction parameters set therein have been used for correcting the distortions caused by the quadrature modulator. The problem is then manufacturability, since each quadrature modulator requires unique parameters, which must be set as early as during the production. In addition, as already stated above, the magnitude of the amplitude imbalance caused by the quadrature modulator depends on the frequency, in which case when the frequency is changed, the fixedly-set correction does not necessarily work any longer. Therefore adaptive signal processing techniques are very desirable methods to correct the distortions. Various adaptive techniques for compensating the amplitude imbalance caused by the quadrature modulator are also known. Often these techniques, however, require the use of a special training signal and/or are dependent on knowing the quadrature transmitter chain gain, the exact value of which can vary from device-specifically.
An object of the present invention is thus to provide a method and an arrangement for compensating for an amplitude imbalance of a quadrature modulator such that no special training signal is needed. It is another object of the invention to provide a method and an arrangement for compensating for an amplitude imbalance of a quadrature modulator such that they are independent of the transmitter chain gain.
The objects of the invention are achieved by a method for compensating an amplitude imbalance of a quadrature modulator comprising: determining a first correlation on the basis of a first modulation signal and an output signal of the quadrature modulator; determining a second correlation on the basis of a second modulation signal and the output signal of the quadrature modulator; producing a compensation signal proportional to the amplitude imbalance on the basis of a ratio of the determined correlations and the first and second modulation signals; and processing at least one of the modulation signals of the quadrature modulator with the compensation signal; wherein the correlations are determined on the basis of unprocessed modulation signals of the quadrature modulator.
The objects of the invention are further achieved by an arrangement for compensating for amplitude imbalance of a quadrature modulator comprising: means for determining a first correlation on the basis of a first modulation signal and an output signal of the quadrature modulator; means for determining a second correlation on the basis of a second modulation signal and the output signal of the quadrature modulator; means for producing a compensation signal proportional to the amplitude imbalance on the basis of a ratio of the determined correlations and the first and second modulation signals; and means for processing at least one of the modulation signals of the quadrature modulator with the compensation signal; wherein the means for determining the correlations are arranged to use unprocessed modulation signals of the quadrature modulator for determining the correlations.
The objects of the invention are further achieved by an arrangement for compensating for amplitude imbalance of a quadrature modulator comprising: a first correlator configured to determine a first correlation on the basis of a first modulation signal and an output signal of the quadrature modulator; a second correlator configured to determine a second correlation on the basis of a second modulation signal and the output signal of the quadrature modulator; means configured to produce a compensation signal proportional to the amplitude imbalance on the basis of a ratio of the determined correlations and the first and second modulation signals; and means configured to process at least one of the modulation signals of the quadrature modulator with the compensation signal; wherein the first and second correlators are configured to use unprocessed modulation signals of the quadrature modulator for determining the correlations.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.